1. Field of the Invention
The present application relates to top-emission white-color organic light-emitting diode (OLED) devices for lighting applications.
2. Description of the Related Art
Organic light-emitting materials offer a very promising field of study for energy efficient lighting applications. Many methods have been proposed to increase the OLED device power efficiency, including modifying materials, device structure, device fabrication techniques, and light outcoupling techniques. A traditional OLED comprises a bottom emission type OLED (BE-OLED), wherein the bottom electrode is a transparent conducting metal oxide, such as Indium-Tin-Oxide (ITO) deposited on top of a transparent substrate, such as glass. Generally, without light outcoupling involved, most of the emitted light in a BE-OLED is trapped inside the device in the form of an organic mode, substrate mode, or plasma mode. Only about 10-30% of the light escapes from the device and contributes to the lighting. Thus, the light trapped in the glass substrate may account for 20% of the total emissive light. This generally requires light extraction in BE-OLEDs to be practically necessary.
Recently, top-emission OLED (TE-OLED) devices, wherein the top electrode (generally, the cathode) is either a semi-transparent metal cathode or a transparent conducting metal oxide like ITO, have been explored. For a semi-transparent top cathode, the microcavity effect may be serious due to a relatively higher reflectance of the metal semi-transparent cathode compared with a transparent ITO cathode. This can lead to selective wavelengths passing through the cathode, contributing to the light output and viewing angle dependence of the emission spectrum. While such a feature may be good for display applications, it can also negatively affect general lighting applications because white-color light emission is desired.
There are many challenging issues in TE-OLED manufacturing, including materials for the bottom reflective anode, the active cells of the light-emitting layers, and the semi-transparent cathode. Also, tuning the light enhancement layer and the light scattering layer, all while further enhancing the power efficiency of TE-OLED to meet various lighting application requirements, invokes large amounts of consideration. Compared to BE-OLED, the efficiency needs of TE-OLED require much more attention in order to meet the light application requirement.